1. Field of the Invention
The present invention is directed to methods and instrumentation of ultrasound imaging in a wide frequency range where the digital beamformer is reconfigurable in terms of number of channels versus frequency range.
2. Description of the Related Art
Digital ultrasound beam formers for medical ultrasound imaging have the last decade become feasible due to improved functionality of analog to digital converters (ADCs) and digital integrated circuit technology. However, the requirements on the beam former in terms of number of channels, frequency bandwidth, signal dynamic range, etc., highly depend on the application and the resolution versus depth penetration required.
The cost of the beam former per channel is dominated by the cost of the ADCs, which increases with number of bits and maximal sampling frequency of the ADC. The requirement for number of bits is determined by the required dynamic range, where blood velocity imaging in the heart puts the strongest requirement on the dynamic range (and number of bits) due to the demanding filtering of the wall signals to retrieve the blood signal for the velocity processing. Non-cardiac imaging requires less dynamic range and number of bits in the ADCs, and an increase in the center frequency and the bandwidth further reduces the dynamic range in the signal and hence the required number of bits. Reducing the transducer array element dimensions also reduces the number of required bits per channel.
It is hence a need for a beam former where the number of channels, dynamic range, and frequency range can be reconfigured for the particular application at hand.
The largest number of channels are found with the phased arrays, where the element pitch is λ/2, where λ=c/f is the wave length of ultrasound in the tissue with ultrasound propagation velocity c (˜1.54 mm/μsec) and f is the ultrasound frequency. With switched linear or curvilinear arrays, the element pitch can be increased to λ-1.5λ, increasing the aperture by a factor 2-3 compared to the phased array with the same number of elements, or allows for a reduction in the number of electronic channels in the beam former with limited increase in the aperture. With the beam axis along the surface normal of the array (no angular direction steering of the beam), one can also do analog summation of the signals for the pair of elements with symmetric location around the aperture center, hence reducing the required number of ADCs by a factor 2, or expanding the number of elements by a factor 2 with a given number of ADCs.
The annular arrays require even less number of delay channels. As the element areas are larger than for the switched arrays, their electrical impedance is proportionally less, and it is practical to parallel couple analog channels for each element of the annular array so that for similar apertures and frequencies one gets about the same number of analog channels for the annular and the switched arrays. This statement specially applies to the annular array design described in U.S. Pat. No. 6,622,562 Sep. 23, 2003, where the outer elements have specially large area.
Manufacturing technology gives a limitation on the lowest pitch of the array elements, where λ/2 pitches are achievable for frequencies up to ˜10 MHz with current transducer array technology. This is hence the highest frequency where the phased array method has been used, while for higher frequencies one is using switched arrays where the lowest manufacturing pitch with current technology allows frequencies up to ˜20-30 MHz. Current experimental manufacturing techniques allow frequencies of switched arrays up to ˜50 MHz.
The annular arrays have the fewest number and hence the largest elements for a given aperture. They therefore allow the use of the highest frequencies, even up to ˜100 MHz with current technology. One should also note that the phased array image is mainly interesting for imaging between ribs and from localized areas, where a highest frequency of ˜10 MHz is adequate, while the image formats of the switched and annular arrays are applicable over the whole frequency range. With some intra-luminal catheter and surgical applications one can see the sector image format of the phased array also being attractive for frequencies above ˜10 MHz. With new transducer technology based on ceramic films or micromachining of silicon (cmut—capacitive micromachined ultrasound transducers), one sees opportunities for manufacturing of phased arrays with center frequencies above ˜10 MHz.
It is hence a need for a beam former that can run a large number of channels for wide aperture phased and linear arrays up to a center frequency f0˜15 MHz, with a less number of channels for frequencies up to 2f0˜30 MHz with switched arrays and annular arrays, and an even less number of channels for frequencies up to 4f0-7f0˜60-100 MHz to be operated with switched and annular arrays.